University of Leicester
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Modelling the seasonal cycle of Uranus’s colour and magnitude, and comparison with Neptune

Version 2 2024-06-07, 16:06
Version 1 2024-02-05, 12:42
journal contribution
posted on 2024-06-07, 16:06 authored by Patrick GJ Irwin, Jack Dobinson, Arjuna James, Nicholas A Teanby, Amy A Simon, Leigh N Fletcher, Michael T Roman, Glenn S Orton, Michael H Wong, Daniel Toledo, Santiago Pérez-Hoyos, Julie Beck

We present a quantitative analysis of the seasonal record of Uranus’s disc-averaged colour and photometric magnitude in Strömgren b and y filters (centred at 467 and 551 nm, respectively), recorded at the Lowell Observatory from 1950 to 2016, and supplemented with HST/WFC3 observations from 2016 to 2022. We find that the seasonal variations of magnitude can be explained by the lower abundance of methane at polar latitudes combined with a time-dependent increase of the reflectivity of the aerosol particles in layer near the methane condensation level at 1 – 2 bar. This increase in reflectivity is consistent with the addition of conservatively scattering particles to this layer, for which the modelled background haze particles are strongly absorbing at both blue and red wavelengths. We suggest that this additional component may come from a higher proportion of methane ice particles. We suggest that the increase in reflectivity of Uranus in both filters between the equinoxes in 1966 and 2007, noted by previous authors, might be related to Uranus’s distance from the Sun and the production rate of dark photochemical haze products. Finally, we find that although the visible colour of Uranus is less blue than Neptune, due to the increased aerosol thickness on Uranus, and this difference is greatest at Uranus’s solstices, it is much less significant than is commonly believed due to a long-standing misperception of Neptune’s ‘true’ colour. We describe how filter-imaging observations, such as those from Voyager-2/ISS and HST/WFC3, should be processed to yield accurate true colour representations.


Planetary Science at Oxford Physics 2019

Science and Technology Facilities Council

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Studies on Planetary Formation and Evolution at Bristol

Science and Technology Facilities Council

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Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration (80NM0018D0004)

European Research Council Consolidator Grant (under the European Union’s Horizon 2020 research and innovation programme, grant agreement no. 723890) at the University of Leicester

Grant number PID2019-109467GB-I00Z, funded by MCIN/670 AEI10.13039/501100011033 and Elkartek KK-2023/00077

This review includes observations made with the NASA/ESA Hubble Space Telescope obtained from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc. (AURA), under NASA contract number NAS 5–26555. These observations are associated with STIS programme(s) GO9035, GO9330, GO12894, GO14113, and WFC3 programmes GO13937, GO14334, GO14756, GO15262, GO15502, GO15929, GO16266, GO16790, GO16995. AS is supported by grants from the Space Telescope Science Institute associated with programme GO13937


Author affiliation

School of Physics & Astronomy, University of Leicester


  • VoR (Version of Record)

Published in

Monthly Notices of the Royal Astronomical Society






11521 - 11538


Oxford University Press (OUP)





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Correction: The publication date of the originally published version of this manuscript was incorrectly given as 12 September 2023, instead of 5 January 2024.



Data Access Statement

The data underlying this article will be shared on reasonable request to the corresponding author. Please note, however, that the Hubble data are publicly available at the Mikulski Archive for Space Telescopes ( under the programme numbers listed in the acknowledgements.

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